After some consideration and tips, I have decided to move my expansion tank from the attic to our insulated garage. There is an old water softener loop in the garage about 12 feet from the hot water heater (water softener was removed years ago by previous owner and loop was left). I have heard that an expansion tank can be added anywhere on the cold side, but I don't know anything about softener loops. There is a cutoff valve on the loop and when shut off it cuts the water throughout the house. Any reason why this wouldn't be a good place to install it? Thanks.
It is best to have the normal main shutoff for the house after the pressure tank. That is, the pump should still have access to the pressure tank when you are working on plumbing (other than working on the pump and pressure tank themselves when you would switch off the pump).
Is there another valve in the basement or elsewhere that could let cold water go to the house and also the water heater without going through the softener loop?
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I thought that it is typical to add the expansion tank just upstream of your water heater with a check valve just upstream of that. This configuration should take care of both thermal expansion and water hammer.